Electrical resonators are used in a variety of electrical circuits to perform a variety of functions. Depending upon the structure and material of the resonator, when an AC signal is applied to the resonator over a broad frequency range the resonator will resonate at specific resonant frequencies. This characteristic allows the resonator to be used, for example, in an electrical filter that is designed to pass only frequencies in a preselected frequency range, or to attenuate specific frequencies.
Resonators are also used in high frequency applications, such as optical communication systems which operate in the GHz range. In these types of applications, resonators are used, for example, to stabilize the frequency of oscillators in repeater modules that are provided along an optical communication transmission line. These types of resonators must exhibit high Q values in order to provide the necessary oscillator frequency stability and spectral purity, and also maintain low phase noise.
There are several types of such high Q resonators known in the art. For example, cavity resonators, coaxial resonators, transmission line resonators and dielectric resonators have all been used in high Q applications. Cavity and dielectric resonators, however, are difficult to mass produce in an efficient manner, because these devices consist of machined parts. There is also significant manual labor involved in assembling the devices and mounting them to circuit boards, as well as in tuning the devices to the desired resonant frequency.
Ceramic coaxial resonators are also relatively expensive to mass produce as they are individually machined and tested to achieve the desired resonant frequency. In surface mount applications, they are typically limited to frequencies less than 5 GHz due to dimensions, parasitics and spurious modes.
Transmission line resonators, typically microstripline, can be easily fabricated along with interconnection traces on a printed circuit board. This technique can provide only low performance resonators. They are low Q, typically <80, and have poor frequency stability with changing temperature resulting from material properties and geometry. Microstripline resonators are also inherently un-shielded and therefore affected by materials and components in proximity to them. Moreover, transmission line resonators are typically large in size, which is a serious issue in the constant drive to miniaturize electronic components.
Dielectric resonators take the shape of a disc or cylinder. Typical 2 GHz dielectric resonators are about one inch in diameter and one-half inch high. Typical 10 GHz dielectric resonators are about 0.25 inches in diameter and 0.1 inches high. This resonator achieves very high Q because of its size and lack of metallic losses, and is capable of providing excellent frequency stabilization in the GHz range. This device, however, tends to occupy too much real estate to be useful in most microelectronic applications particularly when housing requirements are included. In addition, this device must be fully shielded in a housing to prevent interference by and with surrounding components on the circuit board. Moreover, these products are manufactured by iteratively machining and testing until the desired resonant frequency is achieved. Consequently, this known device is also relatively expensive to mass produce and difficult to assemble on a circuit board.
It would be desirable to provide a high Q resonator that can be designed to resonate at a variety of specific resonant frequencies, but at the same time be simple in structure and inexpensive to mass produce using proven materials (e.g., ceramics) and proven microelectronic techniques (e.g., lithography). To date, however, no such resonator exists.